1. Field of the Invention
The present invention concerns synchronous data transmission using a signal which, in the baseband, is formed by a succession of two-level signal elements without overlap, isochronous, of duration T, and able to take one of the four forms defined by the functions: ##EQU2##
2. Description of the Prior Art
Such synchronous data transmission results from a combination of the L biphase and modified biphase codes.
The L biphase code, also known as the "Manchester" code, converts a sequence of binary information available at the frequency 1/T into a transmission signal formed by a succession of two-level signal elements which are isochronous and without overlap, having halfway through their period a transition whose direction depends on the value of the coded binary information.
The modified biphase code converts a sequence of binary information available at the frequency 1/T into a transmission signal formed by a succession of two-level signal elements which are isochronous and without overlap, having two transitions in opposite directions at the end of the first and third quarter-periods, the direction of the first transition depending on the value of the coded binary information.
For the same Baud period 1/T, the signals resulting from the L biphase and modified biphase codes have principal frequency spectrum lobes of the same width 0.2/T with a maximum shifted towards the low frequencies for the L biphase code and towards the high frequencies for the modified biphase code. They have the special feature that they may be considered as double sideband modulations of two Walsh functions in phase quadrature: ##EQU3## and of being combinable with a modulation in phase quadrature to define a code for which the line signal z'(t) may be expressed as follows, ignoring any multiplying factor: ##EQU4## where a.sub.K and b.sub.K are two synchronous streams of binary information to be transmitted of value .+-.1 available at the frequency 1/T and d(t-KT) is a time function having a unity value in the Kth Baud interval and a null value at all other times. This code has the benefit of doubling the information bit rate as compared with the L biphase and modified biphase codes without doubling the bandwidth occupied. Its signal element may take one of the four possible forms defined by the functions: ##EQU5## and which may be distinguished from one another by the values assumed by the levels during their second and third quarter-periods.
With this type of transmission code it is a simple matter to mark in the received signal, in the baseband, the Baud periods of the signal elements to the nearest K.pi./2 by synchronizing on the transitions, although it is more difficult to resolve the K.pi./2 uncertainty. It is not possible to resolve this uncertainty in the same way as the K.pi. uncertainty occurring with the L biphase code, by distinguishing the transitions halfway through the Baud period from those at the ends of the Baud period, by virtue of the fact that the former appear systematically and the latter do not, or in the same way as the K.pi./2 uncertainty occurring with the modified biphase code, based on the systematic absence of transitions halfway through the Baud period, as these two criteria are no longer valid.
The objective of the present invention is to overcome the aforementioned problem with the aid of a simple circuit which is readily implemented in digital form.